The most known shapes of silver halide grains are cubes and octahedrons, which are composed respectively of (100) planes and (111) planes as outer surfaces.
However, in the case of photographic emulsions used in practice, it is difficult to prepare perfect cubes or perfect octahedrons, and generally tetradecahedrons resulting from chipping of apexes of cubes and octahedrons are used, in many cases. In the case of such tetradecahedrons, (111) planes and (100) planes are exposed as outer surfaces. The surfaces of tabular grains having parallel double twinned crystal planes are (111) planes, crystallographically. However, in fact, most contain (100) planes, because the activity of side planes of tabular grains is particularly high. The following publications relate to surfaces of crystals that are composed of (111) planes and (100) planes, JP-B ("JP-B" means examined Japanese patent publication) No. 42738/1980, JP-A ("JP-A" means unexamined published Japanese patent application) No. 142439/1991, and European Patent No. 515894. However, even the emulsions prepared according to the descriptions in these publications result in unsatisfactory photographic properties.
With respect to tabular silver halide grains, their preparation and use techniques are already disclosed, for example, in U.S. Pat. Nos. 4,434,226, 4,439,520, 4,414,310, 4,433,048, 4,414,306, and 4,459,353, and their advantages are known, such as an improvement in sensitivity/graininess relation, as well as an improvement in the efficiency of color sensitization by spectrally sensitizing dyes.
However, it is generally known that, as the added amount of a sensitizing dye is increased, the inherent sensitivity of the emulsion is decreased. Therefore, even when tabular grains having a large surface area are used and a large amount of spectrally sensitizing dyes is added, a desired improvement in sensitivity/graininess cannot be obtained, and the characteristics of tabular grains are not fully exhibited.
On the other hand, in the field of color photographic light-sensitive materials, particularly color reversal light-sensitive materials very often used by professional photographers, color light-sensitive materials high in sensitivity are demanded for sports photographs, wherein high shutter speeds are required, and for photographs for special scenes, including stage photographs, wherein the amount of light needed for exposure is insufficient. However, color photographic light-sensitive materials high in sensitivity are rough in graininess. Therefore, improvement in the relationship of sensitivity/graininess is desired.
The following means exist for increasing the sensitivity of silver halide emulsions: (1) increasing the number of photons to be absorbed into respective grains, (2) increasing the efficiency for converting photoelectrons generated by the absorption of light to silver clusters (latent images), and (3) increasing development activity in order to effectively utilize the produced latent images.
Making the size of grains large is an effective means of increasing the number of photons absorbed in respective grains, but it is not a preferable means in that it is generally accompanied by deterioration of graininess. In order to increase the sensitivity without deterioration of graininess, it is most preferable to increase the efficiency of converting photoelectrons to latent images, i.e. to increase the quantum sensitivity. To increase the quantum sensitivity, it is required to eliminate inefficient processes as much as possible, such as the recombination of photoelectrons with light positive holes and the dispersion of latent images.
As one means of decreasing the recombination of photoelectrons with light positive holes, reduction sensitization has been studied for a long time. For example, tin compounds are disclosed as a useful reduction sensitizer in U.S. Pat. No. 2,487,850, polyamine compounds are disclosed as a useful reduction sensitizer in U.S. Pat. No. 2,512,925, and thiourea dioxide-series compounds are disclosed as a useful reduction sensitizer in British Patent No. 789,823. Further, in "Photographic Science and Engineering," Vol. 23, page 113 (1979), the shapes and properties of silver nuclei formed by various reduction sensitization methods are compared, and, in the methods, dimethylamine borane, stannous chloride, hydrazine, high-pH ripening, and low-pAg ripening are used.
Further, U.S. Pat. Nos. 2,518,698, 3,201,254, 3,411,917, 3,779,777, and 3,930,867 also disclose reduction sensitization methods. Not only the selection of reduction sensitizers but also the design of reduction sensitization methods are described in JP-B Nos. 33572/1982 and 1410/1983.
Further, it is also known that in view of high sensitization, tabular silver halide grains are more advantageous than other grains such as octahedrons, tetradecahedrons, and the like. This is because, since the surface area of tabular silver halide grains per unit volume is large, tabular silver halide grains can absorb a larger amount of a sensitizing dye at the time of spectral sensitization, and they are high in trapping ability to incident light.
In view of the above, methods wherein tabular silver halide grains are subjected to reduction sensitization, to obtain a highly sensitive emulsion, are described, for example, in JP-A Nos. 288145/1991, 355748/1992, and 313282/1993. High sensitization by these methods was studied, and although a considerable increase in sensitivity was positively confirmed, new problems occurred, in that deterioration of graininess and a remarkable change in development progression were also brought about. It was found that, for example, when gold sensitization and chalcogen sensitization were additionally used, and YF colloidal silver was used, the problems became extremely serious, and when tabular grains high in aspect ratio, or silver halide grains having a low iodine content, were used, the problems became conspicuous. Consequently, further technical improvement for solving these problems is desired.
In addition, although silver halide emulsion grains are basically not sensitive to the visible region, they have been caused to adsorb various dyes on the surfaces thereof, in order to get a desired spectral sensitivity. In particular, in comparison with spherical grains, tabular emulsion grains can absorb dyes on their principal planes, to increase the light absorption ratio. Therefore they are very advantageous in spectral sensitization. In particular, color photographic light-sensitive materials are composed of emulsion grains having various sizes, and for color photographic light-sensitive materials, there is need for a technique of preparing emulsion grains composed of tabular grains wherein the above advantages are taken and the side planes are thin in comparison with the principal planes, i.e., the aspect ratio is high.
In some cases, a hitherto developed dye addition method, as disclosed in JP-A No. 318839/1992, makes it possible to apply dyes having desired spectral absorption to tabular grains having various sizes without impairing the graininess/sensitivity ratio. However, according to the investigation done by the inventors of the present invention, it has been made clear that regardless of various dye addition methods, in comparison with thick grains, thin tabular grains have a problem that the extent of the graininess to the sensitivity (hereinafter referred to as graininess/sensitivity ratio) is poor.